Beginners Guide to Troubleshooting Common SELinux Issues

It is important to understand what actions you must take when SELinux prevents access to files on a server that you know should be accessible. Use the following steps as a guide to troubleshooting these issues:

  1. Before thinking of making any adjustments, consider that SELinux may be doing its job correctly by prohibiting the attempted access. If a web server tries to access files in /home, this could signal a compromise of the service if web content is not published by users. If access should have been granted, then additional steps need to be taken to solve the problem.
  2. The most common SELinux issue is an incorrect file context. This can occur when a file is created in a location with one file context and moved into a place where a different context is expected. In most cases, running restorecon will correct the issue. Correcting issues in this way has a very narrow impact on the security of the rest of the system.
  3. Another remedy for overly restrictive access could be the adjustment of a Boolean. For example, the ftpd_anon_write boolean controls whether anonymous FTP users can upload files. You must turn this boolean on to permit anonymous FTP users to upload files to a server. Adjusting booleans requires more care because they can have a broad impact on system security.
  4. It is possible that the SELinux policy has a bug that prevents legitimate access. Since SELinux has matured, this is a rare occurrence. When it is clear that a policy bug has been identified, contact Red Hat support to report the bug so it can be resolved.

Monitoring SELinux Violations

Install the setroubleshoot-server package to send SELinux messages to /var/log/messages. setroubleshoot-server listens for audit messages in /var/log/audit/audit.log and sends a short summary to /var/log/messages. This summary includes unique identifiers (UUID) for SELinux violations that can be used to gather further information. The sealert -l UUID command is used to produce a report for a specific incident. Use sealert -a /var/log/audit/audit.log to produce reports for all incidents in that file.

Consider the following sample sequence of commands on a standard Apache web server:

[[email protected] ~]# touch /root/file3
[[email protected] ~]# mv /root/file3 /var/www/html
[[email protected] ~]# systemctl start httpd
[[email protected] ~]# curl http://localhost/file3
<title>403 Forbidden</title>
<p>You don't have permission to access /file3 on this server.</p>

You expect the web server to deliver the contents of file3 but instead it returns a permission denied error. Inspecting both /var/log/audit/audit.log and /var/log/messages reveals extra information about this error.

[[email protected] ~]# tail /var/log/audit/audit.log
...output omitted...
type=AVC msg=audit(1392944135.482:429): avc:  denied  { getattr } for
pid=1609 comm="httpd" path="/var/www/html/file3" dev="vda1" ino=8980981
tcontext=unconfined_u:object_r:admin_home_t:s0 tclass=file
...output omitted...
[[email protected] ~]# tail /var/log/messages
...output omitted...
Feb 20 19:55:42 host setroubleshoot: SELinux is preventing /usr/sbin/httpd
from getattr access on the file . For complete SELinux messages. run
sealert -l 613ca624-248d-48a2-a7d9-d28f5bbe2763

Both log files indicate that an SELinux denial is the culprit. The sealert command that is part of the output in /var/log/messages provides extra information, including a possible fix.

[[email protected] ~]# sealert -l 613ca624-248d-48a2-a7d9-d28f5bbe2763
SELinux is preventing /usr/sbin/httpd from getattr access on the file .

*****  Plugin catchall (100. confidence) suggests   **************************

If you believe that httpd should be allowed getattr access on the file by default.
Then you should report this as a bug. You can generate a local policy module to allow this access.
allow this access for now by executing:
# grep httpd /var/log/audit/audit.log | audit2allow -M mypol
# semodule -i mypol.pp

Additional Information:
Source Context                system_u:system_r:httpd_t:s0
Target Context                unconfined_u:object_r:admin_home_t:s0
Target Objects                 [ file ]
Source                        httpd
Source Path                   /usr/sbin/httpd
Port                          [Unknown]
Host                          servera
Source RPM Packages           httpd-2.4.6-14.el7.x86_64
Target RPM Packages
Policy RPM                    selinux-policy-3.12.1-124.el7.noarch
Selinux Enabled               True
Policy Type                   targeted
Enforcing Mode                Enforcing
Host Name                     servera
Platform                      Linux servera 3.10.0-84.el7.x86_64 #1
                              SMP Tue Feb 4 16:28:19 EST 2014 x86_64 x86_64
Alert Count                   2
First Seen                    2014-02-20 19:55:35 EST
Last Seen                     2014-02-20 19:55:35 EST
Local ID                      613ca624-248d-48a2-a7d9-d28f5bbe2763

Raw Audit Messages
type=AVC msg=audit(1392944135.482:429): avc:  denied  { getattr } for
pid=1609 comm="httpd" path="/var/www/html/file3" dev="vda1" ino=8980981
tcontext=unconfined_u:object_r:admin_home_t:s0 tclass=file

type=SYSCALL msg=audit(1392944135.482:429): arch=x86_64 syscall=lstat
success=no exit=EACCES a0=7f9fed0edea8 a1=7fff7bffc770 a2=7fff7bffc770
a3=0 items=0 ppid=1608 pid=1609 auid=4294967295 uid=48 gid=48 euid=48
suid=48 fsuid=48 egid=48 sgid=48 fsgid=48 tty=(none) ses=4294967295
comm=httpd exe=/usr/sbin/httpd subj=system_u:system_r:httpd_t:s0 key=(null)

Hash: httpd,httpd_t,admin_home_t,file,getattr

The Raw Audit Messages section of the sealert command contains information from /var/ log/audit.log. To search the /var/log/audit.log file use the ausearch command. The m searches on the message type. The -ts option searches based on time.

[[email protected] ~]# ausearch -m AVC -ts recent
time->Tue Apr  9 13:13:07 2019
type=PROCTITLE msg=audit(1554808387.778:4002):
type=SYSCALL msg=audit(1554808387.778:4002): arch=c000003e syscall=49
success=no exit=-13 a0=3 a1=55620b8c9280 a2=10 a3=7ffed967661c items=0
ppid=1 pid=9340 auid=4294967295 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0
sgid=0 fsgid=0 tty=(none) ses=4294967295 comm="httpd" exe="/usr/sbin/httpd"
subj=system_u:system_r:httpd_t:s0 key=(null)
type=AVC msg=audit(1554808387.778:4002): avc:  denied  { name_bind }
for  pid=9340 comm="httpd" src=82 scontext=system_u:system_r:httpd_t:s0
tcontext=system_u:object_r:reserved_port_t:s0 tclass=tcp_socket permissive=0